Method of coating wide-angle cathode ray picture tube envelopes

ABSTRACT

AN IMPROVED METHOD OF EVAPORATING METAL COATINGS ON THE MAJOR INNER SURFACE OF WIDE-ANGLE CATHODE RAY PICTURE TUBE ENVELOPES UTILIZES A FRUSTO-CONICAL EVAPORATOR COMPRISING, FOR EXAMPLE, A FRUSTO-CONICAL HEATING COIL AND A FRUSTO-CONICAL METAL SLEEVE. WITH THIS METHOD, THE RATIO OF THE METAL THICKNESS AT THE EDE TO THE METAL THICKNESS AT THE CENTER OF THE ENVELOPE FACEPLATE IS SUBSTANTIALLY GREATER THAN THE RATIO PREVIOUSLY OBTAINED. IN THE MANUFACTURE OF BLACK-AND-WHITE TELEVISION PICTURE TUBES, APPLICATION OF THE FRUSTO-CONICAL EVAPORATOR SHOULD SUBSTANTIALLY REDUCE THE AMOUNT OF TUBE REJECTS.   D R A W I N G

Jung 15 1971 SALVETER, JR v 3,582,394 v METHOD OF COATING WIDE-ANGLE CATHODE RAY PICTURE TUBE ENVELOPES I Filed Dec. 16, 1968,

United States Patent O 3,582,394 METHOD OF COATING WIDE-ANGLE CATHODE RAY PICTURE TUBE ENVELOPES Robert E. Salveter, Jr., Marion, Ind., assignor to RCA Corporation Filed Dec. 16, 1968, Ser. No. 783,946 Int. Cl. B4441 5/12, 5/00 U.S. Cl. 117-97 3 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to the manufacture of wideangle cathode ray picture tubes, and particularly to improvements in the method of evaporating metal coatings on the major inner surfaces of the tube envelopes.

In certain types of cathode ray picture tubes such as are used in black-and-white television receivers, it has proved advantageous to coat the inner surface of the tube faceplate with a thin, light-reflecting, electronpervious layer of metal such as aluminum. The metallic layer intensifies the useful light directed toward an observer by reflecting light emitted from the faceplate phosphor screen in the direction of the tube interior. Furthermore, the metal coating absorbs the negative ion component of the scanning electron beam and prevents its destructive bombardment of the phosphor screen. Other well known advantages are also achieved with an aluminum coating on the inner surface of the faceplate.

It is desirable also to coat the inner surface of the tube funnel with a thin, conducting layer of metal such as aluminum. For convenience in the manufacture of cathode ray picture tubes, the inner surfaces of the tube faceplate and the tube funnel are usually coated at the same time.

The various techniques usually used for applying the metallic layer to the faceplate involve first forming on the phosphor screen an organic film having a bright surface, applying the metallic layer to the bright surface of the organic film, and then baking out the organic film to remove the same. The organic film is formed originally to fill the surface irregularities of the screen and thereby to provide a smooth foundation layer for the metal coating.

The metal coating is commonly applied to the organic film by evaporating aluminum in vacuum from a conventional basket evaporator located-on the bulb axis. The basket evaporator comprises a short hollow cylinder of aluminum charge seated in a basket-shaped tungsten heater. With this method, approximately equal amounts r 3,582,394 Ice Patented June 1, 1971 of aluminum are propagated in all directions from the evaporator. However in wide-angle cathode ray picture tubes, the straight-line distance between the evaporator and the center of the faceplate is much less than one half the maximum dimension of the faceplate. Hence the evaporator is much closer to the center of the phosphor screen than it is to the edge of the screen. As a result, the metal coating is much thicker at the center than it is at the edge of the phosphor screen.

A wide-angle tube manufactured for use in high anode voltage receivers may be rejected for having too-thin a metal coating at the edge of the tube faceplate. That is, the edge coating may be too thin to provide suflicient light reflection. On the other hand, a wide-angle tube manufactured for use in low anode voltage receivers may have too thick a metal coating at the center of the faceplate. That is, the center coating may be too thick to permit sufficient beam penetration. Typically, the ratio of aluminum thickness at the edge to aluminum thickness at the center of a tube faceplate is only about 0.35 for a -degree deflection, 12-inch diagonal, rectangular cathode ray picture tube.

Accordingly, an object of the present invention is to provide a substantially improved method of evaporating metal coatings on the major inner surfaces of wide-angle cathode ray picture tube envelopes whereby the ratio of the aluminum thickness at the edge to the aluminum thickness at the center of a tube faceplate is at least 30 percent higher than the ratio obtained with a conventional basket evaporator located on the bulb axis.

SUMMARY OF THE INVENTION Briefly, the method of the invention utilizes a frustoconical evaporator in place of a conventional basket evaporator to apply a metal coating on the inner surfaces of the faceplate and funnel of a wide-angle cathode ray picture tube. The frusto-conical evaporator may comprise a frusto-conical heating coil and a frusto-conical metal sleeve positioned thereon in such manner that the edge of the faceplate being coated is less shielded from the lower surfaces of the evaporator coil. Thus the ratio of the metal thickness at the edge to the metal thickness a the center of the faceplate phosphor screen is substantially improved over the ratio previously obtained.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is described in greater detail in connection with the accompanying sheet of drawings, wherein:

FIG. 1 is a side view, partly in axial section, of a typical wide-angle cathode ray picture tube envelope, showing both the location and orientation of a frustoconical evaporator;

FIG. 2 is a side view of a frusto-conical heating coil over which a sleeve is mounted to complete the evaporator;

FIGS. 3 and 4 are side and top views, respectively, of the frusto-conical metal sleeve; and

FIG. 5 is a graph showing the relative directionalities of evaporation from a frusto-conical evaporator and a conventional basket evaporator.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 depicts a wide-angle cathode ray picture tube envelope 1 of the form and dimensions common to the black-and-white television tube art. The glass envelope 1 comprises a large-diameter curved faceplate 3 joined at its periphery to the larger end of a funnel 5, the distance between the smaller end of funnel 5 and the center of faceplate 3 being less than one half the maximum dimension of the faceplate 3. For round picture tubes, the maximum dimension is the diameter of faceplate 3, and for rectangular picture tubes, the maximum dimension is the diagonal of faceplate 3. The inner surface of faceplate 3 contains a black-and-White television phosphor screen 7 formed in a typical manner of the art. A frustoconical evaporator 9, for applying a metal coating on the surface of phosphor screen 7 and the inner surface of funnel 5, is positioned on the axis of the tube, Within funnel 5, and near the smaller end thereof.

The frusto-conical evaporator 9 shown in FIG. 1 comprises a frusto-conical heating coil 11 over which is mounted a frusto-conical metal sleeve 13. The coil 11, shown in detail in FIG. 2, may be a stranded wire of refractory metal such as tungsten. The sleeve 13, shown in detail in FIGS. 3 and 4, is made of the metal to be evaporated, typically aluminum. The thickness of the sleeve in FIGS. 3 and 4 can be varied to evaporate a desired amount of material. Returning again to FIG. 1, the frusto-conical evaporator 9 is positioned with its small end facing the faceplate 7 so that the outer surface of the frusto-conical sleeve 13 faces outwardly and forwardly toward the faceplate 3. For example, the evaporator sleeve 13 and coil 11 are shown in the drawing as having a taper angle of 15.

In order to apply the metallic layer to the phosphor screen 7 in FIG. 1, an organic film is first formed on screen 7, as discussed previously. Thus the major inner surfaces of the tube envelope 1, i.e., of the phosphor screen 7 and the funnel 5, are ready for the metal coating. Envelope 1, including the frusto-conical evaporator 9, is evacuated by connecting the smaller end of the funnel 5 to a vacuum pumping system (not shown). Then, the frusto-conical sleeve 13 is heated by the frusto-conical coil 11. The sleeve 13 is heated at a temperature and for a time sufficient to evaporate the required amount of metal of sleeve 13 onto the major inner surfaces of the envelope 1.

Heretofore, a main concern with the metal coating process has been the obtainable ratio of the metal thickness at the edge to the metal thickness at the center of the tube faceplate 3 in FIG. 1. Unsatisfactorily-low ratios, typically about 0.35 for a IlO-degree deflection, 12-inch diagonal, rectangular cathode ray picture tube, have been a cause of tube rejects as discussed previously. However, with the present invention incorporating the frusto-conical evaporator 9, the ratio of the aluminum thickness at the edge to the aluminum thickness at the center of the tube faceplate 3 is at least 30-percent higher than the ratio obtained with a conventional basket evaporator located on the bulb axis. In the case of rectangular tubes, the edge thickness is measured at the corners of the faceplate.

This improved result is explained by the shape of the frusto-conical evaporator 9 and the manner of positioning of evaporator 9 within the tube funnel 5. When the evaporator sleeve 13 is heated by the heating coil 11, molten aluminum from sleeve 13 is distributed over coil 11. But because of the gravitational effect, the molten aluminum is concentrated on the lower turns of heating coil 11. The center of phosphor screen 7, located directly above the evaporator during the metal coating process, is somewhat shielded by the upper turns of the coil 11 from the aluminum evaporating from the lower turns of coil 11; whereas the edge of screen 7 is less shielded from the lower turns of the coil 11.

A comparison can be made of the results of evaporating aluminum from a frusto-conical evaporator and a conventional basket evaporator. In each of two examples, measurements were made of the thickness of the metallic layer deposited on the phosphor screen of a llO-degree deflection, 12-inch diagonal, rectangular cathode ray picture tube envelope. For the metal coating produced by a frustoconical evaporator having a frusto-conical aluminum sleeve of 77-milligram mass, the average edge thickness was 785 angstroms, the center thickness was 1,575 angstroms, and, therefore, the average edge-to-center thickness ratio was approximately 0.50. For the metal coating produced by a conventional basket evaporator having a hollow cylindrical aluminum charge also of 77-milligram mass, the average edge thickness was 650 angstroms, the center thickness was 1700 angstroms, and, therefore, the average edge-to-center thickness ratio was approximately 0.38. Thus, the average edge-to-center thickness ratio obtained with a frusto-conical evaporator was approximately 32-percent higher than the ratio obtained with a conventional basket evaporator. For larger diagonal wideangle cathode ray picture tube envelopes, the improvement in the metal coating edge-to-center thickness ratio is even greater.

A graphical comparison of the directionalities of propagation from a frusto-conical evaporator and a conventional basket evaporator is presented in FIG. 5, wherein the relative thickness of metal theoretically deposited on the inside surface of a spherical envelope by either evaporator located at the center of the sphere is plotted vs. azimuthal variation of the surface position from the evaporator axis. Curve A applies to the frusto-conical evaporator, and curve B applies to the conventional basket evaporator. The figure illustrates the relative decrease in metal thickness obtainable at the center of a curved faceplate and therefore the relative increase in edge-to-center thickness ratio possible with a frnsto-conical evaporator.

In an alternative form of the frusto-conical evaporator, not illustrated, a frusto-conical heating coil of refractory meal, typically tungsten, may be dipped directly into a bath of the molten metal to be evaporated, typically aluminum. The evaporator comprising the metal-coated frusto-conical heating coil would then be positioned within the funnel in the same manner as the evaporator comprising the uncoated frusto-conical heating coil and the frusto-conical metal sleeve. The steps of the method of evaporating the metal coating would thereafter be the same.

What is claimed is:

1. The method of coating the major inner surfaces of a wide-angle cathode ray picture tube envelope with a metal layer, comprising the steps of:

(a) providing a glass envelope comprising a large diameter curved faceplate joined at its periphery to the large end of a funnel, the distance between the small end of said funnel and the center of said faceplate being less than one half the maximum dimension of said faceplate;

(b) positioning a frusto-conical evaporator sleeve having a small but substantial taper angle and a predetermined weight of coating metal within said funnel, near the smaller end thereof, with the small end of the sleeve facing the faceplate so that the outer conical surface of the sleeve faces outwardly and forwardly toward said faceplate; and

(c) heating said sleeve at a temperature and for a time suflicient to evaporate the metal thereof onto the inner surfaces of said faceplate and funnel.

2. The method of coating the major inner surfaces of a wide-angle cathode ray picture tube envelope with a metal layer, comprising the steps of:

(a) providing a glass envelope comprising a largediameter curved faceplate joined at its periphery to the large end of a funnel, the distance between the small end of said funnel and the center of said faceplate being less than one-half the maximum diameter of said faceplate;

(b) positioning a frusto-conical evaporator sleeve of coating metal having a predetermined weight within said funnel, near the smaller end thereof, with the 6 small end of the sleeve facing the faceplate so that References Cited the outer conical surface of the sleeve faces out- UNITED STATES PATENTS wardly and forwardly toward said faceplate; angl (c) heating said sleeve at a temperature and for a time g g sufficient to evaporate the metal thereof onto the 5 3246626 4/1966 g 8:49

inner surface of said faceplate and funnel, by means of a frusto-conical coil of a more refractory metal, ALFRED L LEAVITT Primary Examiner having a taper angle substantially the same as that v of said sleeve, over which said sleeve is mounted. WHITBY Asslstant Exammer 3. The method of claim 2, wherein said metal sleeve 10 U S Cl XR is of aluminum, said coil is of tungsten, and said taper angle is about 15= 117-107.2R, 124C, 160; 118-48 

